There are no official rules for this, and there never will be, because

Under absolutely no circumstances should rules be used for this ever

A PC–PC seduction attempt succeeds if, and exactly as much as, the target’s player says it does. No other answer is appropriate. No player should ever be forced “by the rules” into roleplaying a seduction, or even having a fade-to-black followed by them roleplaying a character who had been seduced. Your “guess [...] that the DM CAN take away players' agency under the right circumstances” may well be true, but this is never going to be the right circumstance.

And yes, I have read your question and I understand you’re perfectly comfortable to use dice for this. I am arguing that this is, even in that circumstance, a mistake. It sets an extremely uncomfortable precedent, one that is not appropriate to a game of Dungeons & Dragons. There are game systems where this kind of thing is part-and-parcel with the system, exactly what players should expect when they agree to play. But D&D is emphatically not one of them. Which is, again, why rules for this will never see print.

Almost all groups I have played with have actually refused actual skill or ability checks for any PC–PC social interaction. Rolling Diplomacy to persuade a fellow PC, rolling Intimidate to scare them, these are opportunities for one player to dictate another’s character. That is not, in my experience, considered appropriate even in quite banal circumstances. A seduction is anything but.

So roleplay it. Decide for yourself what you think is an appropriate reaction for your character, that you (and the table) are comfortable with, and do that. Don’t leave it up to the dice. Decide for yourself just how much Cha 18 matters to your character. This is not an appropriate place to insert randomness.

This is a really important question because it lies at the heart of what makes a role-playing game a role-playing game and specifically makes D&D D&D. So I will give the answer first and then the explanation.

The player decides what their character wants to do.

The rule is the fundamental rule of D&D to which all other rules are merely clarifications and guidance (my emphasis):

1. The DM describes the environment.




2. The players describe what they want to do.




3. The DM narrates the results of the adventurers’ actions.

Now it doesn't matter if the seduction attempt is from an NPC or another PC - The players describe what they want to do.

This is at the heart of player autonomy which is at the heart of any role-playing game - its your character so you decide what it thinks and does. Nobody else gets a look in: not another player, not a random person on the street, not the Secretary General of the United Nations and certainly not the Dungeon Master.

For the specific example, you decide if you want to say yes, if you want to say no or if you want the dice to decide and that decision is yours alone and sets no precedent.

The only exception to this is when magic is involved and even then magical effects only do what they say they do. For example, if the seduction attempt was accompanied by a Suggestion spell, the suggestion would need to be "reasonable" (which it may not be for all sorts of reasons e.g. sexual orientation, sexual preference, prior monogamous commitment, prior relationship, its just creepy for the player [which it certainly is] etc.) and, if it were, you "pursue the course of action ... described to the best of [your] ability."

I also note that "my guess is that all of my party agrees that the DM CAN take away players' agency under the right circumstances" does not follow from "Nobody, myself included, was against it". Agreeing to a proposed course of action is vastly different from having a course of action imposed on you.

Your sub-questions

What do the rules say about how to handle this situation?

See above.

What ability/attribute/whatever roll is required?

Assuming you want this to be decided by rolling (and I personally wouldn't) there are no specific rules for this.

My initial though was that this might represent some type of contest, however, on being challenged on this I think the better mechanic would be to treat it as a simple Charisma (Persuasion) check against a DC set by the target player - after all, they are the ones who know if being seduced would be Easy, Medium, Hard or Nearly Impossible.

Do the rules say how long a seduction attempt should take and what situational bonuses (or negatives) to apply to it?

The normal advantage/disadvantage mechanic would seem to apply.

To what point do the rules allow your DM to "take control" of your character during this seduction check?

See above.

A final comment

Consenting adults around a gaming table (or anywhere else) can engage in whatever fantasies they wish, however, the idea that it is appropriate for one human being to make unwanted and unwelcome sexual advances to another is both unethical and, in many circumstances, illegal. Being good looking or charming is not an excuse.

The DM definitely should not take away players' agency about who their characters are having sex with. You've told us that you were okay with it in this case, but it sets a dangerous precedent. Players might feel really, really uncomfortable with being told that someone is having sex with their character, and this might not be visible at the table. It's better if the DM doesn't get involved in this decision.

If you think your paladin was successfully seduced, you can say that; if you think he wasn't, you can say that. If you'd like to let the dice decide, you're perfectly within your rights to turn to the rogue player and say: "I think, if you rolled a 15 or better on a Persuasion check, that would let your rogue seduce my paladin."

You could make up more elaborate rules if you wanted, but I don't think it would add much to the game.

This is bad karma. Very bad karma.

RPGs are nothing without player agency. Are there edge cases in 5e, like charm person? Yeah, although I heavily de-emphasize such things in my games. Without the players deciding their own actions, they're not really players, they're just spectators. For that reason alone, many, many GMs exempt the PCs from being the targets of, e.g., bluff checks, persuasion attempts, seduction attempts, and the rest. (There's no special counter-exemption for when a PC wants to do that to another PC.)

But this isn't just a question of agency, this is a question of sexual agency. People can be (and have absolutely every right to be) extremely sensitive to this. Even through the "filter" of an RPG. (Although an RPG can be a very direct and visceral filter, hence the scare quotes.) Even when it's someone else's character... Because it is setting a precedent.

Let's look at the rules under the assumption of good faith

Based on your opening paragraph, all of your players are comfortable with "sexytimes" role play in D&D. What you are asking for is something mechanical. Seduction and such isn't a thing D&D handles well mechanically.

Short Version

Brief responses to your questions:

1. 
   

   What do the rules say about how to handle this situation?

   
   
   

   There aren't any, for seduction, but you can shoehorn a few into this situation if your whole table is bound and determined to do so.

   
   


2. 
   

   What ability/attribute/whatever roll is required?

   
   
   

   None is required, but if you all want to go down this route, then Charisma (Persuasion) is the closest ability check to Seduction. Beware ... there is a can of worms being opened here.

   
   
   

   But wait! One can make the case that Seduction could just as easily be Intimidation, Deception, and / or Performance rather than Persuasion, or even a combination of some or all of them.

   
   


3. 
   

   Do the rules say how long a seduction attempt should take and what
   situational bonuses (or negatives) to apply to it?

   
   
   

   No, the rules are silent on seduction, although some magical spells or creature abilities can have a similar effect. (See MM, p. 285, Succubus, Charm). The core answer to this question is that "seduction is not handled by D&D 5e rules; magical effects may have a similar result, but that still needs to be handled with care by the players at the table."

   
   


4. 
   

   To what point do the rules allow your DM to "take control" of your character during this seduction check?

   
   
   

   They don't. As with item 3 above, the rules are silent about this. If magical effects are involved, and a failed save, there is some "loss of control" by the PC until the save is rolled, or the magical effect ends. For an ability check, you can accurately say that once the dice are rolled, the DM narrates the result (Basic Rules, p. 3). That said, when it comes to interpersonal role playing, the less the DM has to say about this the better.

   
   
   

   Why should the DM interfere with the role play of two player characters?

   
   
   

   The DM makes decisions for all of the NPC's; the players make decisions for the PCs. If you all are fine with the DM taking charge of your characters, and reducing your agency, that's for the group at the table to decide. I strongly recommend against that. Why? You are in the game to be a player, not a spectator. You, the players, make the choices and decisions for your characters. The DM has the rest of the campaign world to handle and make choices for.

   
   

Amplification on the above: ability checks are not magical spells

1. 
   

   One of the players is playing an extremely attractive female character. She's a 1st level rogue with 18 CHA (maxed out, half-elf and the DM allowed it to get 1 bonus point in exchange for a pitiful 6 STR) who is well aware of her charming beauty and willing to use it without any kind of moral sense.

   
   
   

   In the rules for this edition, Charisma is not a measure of physical attractiveness; it is a measure of the power of personality and personal magnetism.

   
   
   
   
   
   • Aside: this edition of D&D chose not to fall into the trap that the 1e AD&D Unearthed Arcana chose to dive into, which was the addition of Comeliness, or physical beauty, as a character trait. There have been some lessons learned over the years. This is one of them.
     
     
     
     If you all, as a table, want Charisma to point towards beauty feel free to do so. The game's rules don't cover that.
   
   
   
   

   
   

   Charisma

   
   
   

   
   
   • Measures: Confidence, eloquence, leadership Important for: Leaders and diplomatic characters (p. 8, Basic Rules)
   
   
   • A character with high Charisma exudes confidence, which is usually mixed with a graceful or intimidating presence. A character with a low Charisma might come across as abrasive, inarticulate, or timid. (Basic Rules, p. 9)
   
   
   • Charisma, measuring force of personality (Basic Rules, p. 57)
   
   

   
   

   
   
   

   Under Ability Checks, we find

   
   
   

   
   

   
   
   • 
     Charisma Deception / Intimidation / Performance / Persuasion (Basic Rules, p. 58)
   
   
   • Charisma measures your ability to interact effectively with others. It includes such factors as confidence and eloquence, and it can represent a charming or commanding personality. (Basic Rules, p. 62)
   
   
   • Other Charisma Checks. The DM might call for a Charisma check when you try to accomplish tasks like the following:
     
     • Find the best person to talk to for news, rumors, and gossip
     
     • Blend into a crowd to get the sense of key topics of conversation (Basic Rules, p. 62)
   
   

   
   

   
   
   

   Unless you all, as a table, agree that Seduction is a form of Persuasion (or Intimidation, Deception, etc) then there is no mechanic for Seduction. All of the above underlines the following point: D&D 5e isn't mechanically built to handle seduction, unless a magical effect (like the one a Succubus uses) is in play - and that gets a saving throw.

   
   


2. 
   

   Last time she tried to seduce my character.

   
   
   

   This is a PvP interaction. Is your table already good with PvP? If yes, proceed. If no, all stop, simply say "No thanks" and play on.

   
   
   

   But, if you all do agree that Seduction is a viable PvP event, and that you are interested in playing this out, then you can have a contested ability check. From "Contests" (Basic Rules, p. 58. Same words in the PHB).

   
   
   

   
   

   ... special form of ability check, called a contest. {emphasis
   mine}
   
   Both participants in a contest make ability checks
   appropriate to their efforts. They apply all appropriate bonuses and
   penalties, but instead of comparing the total to a DC, they compare
   the totals of their two checks. The participant with the higher check
   total wins the contest. That character or monster either succeeds at
   the action or prevents the other one from succeeding. If the contest
   results in a tie, the situation remains the same as it was before the
   contest. Thus, one contestant might win the contest by default. If two
   characters tie in a contest to snatch a ring off the floor, neither
   character grabs it. In a contest between a monster trying to open a
   door and an adventurer trying to keep the door closed, a tie means
   that the door remains shut

   
   

   
   


3. 
   

   I'm playing a male (kind of, but this is for another post) paladin. He's not an "asexual" character, but he's very focused on his goals - he swore an oath of vengeance - and perceives such things like carnal pleasure as useless distractions.

   
   
   

   The simple way to mechanically apply this is to (1) give this Paladin character advantage on the contest, based on his normal modus operandi, and (2) if I were the DM, also give the initiator disadvantage given how little interest your character has in such things.

   
   
   

   But to be honest with you, that's roll playing, not role playing. Your character should be able to say "No thanks, I am not interested" and that's the end of it. It's the course of action I'd recommend; all of the rest of this answer is based on your stating in your question that "your entire table really wants to go there." Over the years, I have found that at a certain point, person-to-person intimate or sexual role play needs to go off screen. The players who are not involved are reduced to spectating at best, or idly waiting for their turn to do something ... and sometimes they'll feel uncomfortable with that situation arising during play.

   
   
   

   
   

   Advantage/Disadvantage
   
   Sometimes an ability check, attack roll, or saving throw is modified by special situations called advantage and disadvantage. Advantage reflects the positive circumstances surrounding a d20 roll, while disadvantage reflects the opposite.

   
   

   
   
   

   You asked for rules? There are some rules that you can apply but it's pretty clunky. D&D 5e isn't built very well to handle this mechanically; your players and their own role playing styles and tastes will overwhelm this if you want to role play it. And if you do that, who needs to roll any dice?

   
   
   

   
   

   When this little conflict escalated, the DM ruled that the seduction
   attempt deserved a die roll. Nobody, myself included, was against it,
   so my guess is that all of my party agrees that the DM CAN take away
   players' agency under the right circumstances. That said, none of us
   has any idea how to make the roll, but that wasn't a problem since I
   decided to do what she was asking as it was perfectly in line with my
   character's behaviour.

   
   

   
   
   

   You were interested in a mechanical answer. You now have one.

   
   
   

   I still recommend against it since roll playing this will rob both you and the high charisma character of some role playing opportunity. If there is a successful seduction / persuasion check made, I further recommend that the two characters involved "fade to black" as they wander off to - off screen - resolve that interaction. The DM has other players, and needs to return the spotlight to the other PCs.

   
   
   

   There are other RPG's that better handle this kind of interaction and play style, if your table is generally comfortable with role playing that kind of interaction.

   
   
   

   I also suggest that you review this answer about sexytimes RPG approaches.

   
   

What was that can of worms we were talking about?

The can of worms is the risks to the social contract that exists at a table, be it formal, informal, or a bit of both. Any player may have a certain limit, along the lines of "I don't want to have XYZ in games I play" where XYZ is anything that makes the player uncomfortable or get grossed out. (thanks, @Trish) A handy tool for any table is the X-card. I suggest that your group consider using it just in case this idea that you have takes you to some awkward or uncomfortable places.

It's OK To Roll For This

I actually like using some randomness in situations like this when it's my PC as the subject. If I feel like my PC has a strong opinion, I'll go with the roleplay solution, but when something comes out of nowhere and I reflect on my in-character mind and I don't see strong guidance, I might decide on my own a die roll is merited. This adds an interesting texture to the fiction; I know I fall into ruts and overthink character actions and it adds some surprise and complication to have unplanned things happen.

Sure, "it's not a defined part of D&D 5e" - but I've played more than 100 RPGs in my life so I don't really care about details like that. The idea of completely non-mechanized player agency is NOT a requirement of RPGs or of fun. As someone mentioned here (now deleted and I don't see it?) the game Monsterhearts has rules for this, noting "you don't get to control what turns you on or who you fall in love with." This thread of gaming dates back to Greg Stafford's seminal design in Pendragon (1985) where characters have Virtues and Vices that one must roll against to avoid giving in to them. Many games have some accommodation for you not always being in 100% mastermind control of your thoughts and actions.

So when people tell you "RPGs can't/don't/shouldn't do that," they are demonstrably incorrect, and they are just saying "but I like playing this other way." We welcome all playstyles here, so don't let anyone make you feel bad about daring to subject your character's mind to the whim of the dice.

Certainly, since this approach is not customary in D&D, if the DM wanted to make it an enforced thing I'd think it would be prudent to have a discussion to make sure the whole group's on the same page about it. But you don't need DM/group buy-in at all to do so if you want to roll to determine your own character's reaction. It's a role-playing technique to keep in your toolbox.

How To Roll For This

How to do it, you ask? When I decide to make a roll of this sort, I'll usually restate it in the form of one of my abilities or resistances, and do it against a somewhat arbitrary DC informed by relevant attributes of the NPC (in this case Charisma, maybe a penalty because my character's generally not into that, whatever I think is appropriate, not taking more than 5 seconds to decide). I'll roll, and then - and yes, D&D doesn't "do this" - be guided by the degree of success or failure. Let's use your scenario from the question. If I arbitrarily set a 12 (+4 CHA, -2 for my character's opinion of her) and roll a 12, then I'll roleplay it as "Well... Maybe..." and make her work for it more. If I set a 12 and roll a natural 20, then it's on. If I roll something down in the 1-2 range, then I take offense and chuck a chamberpot at the pointy-eared trollop. The trick is to let the roll fill in where your conception of the character's mind leaves off but then flow back into "things they'd do" with the responses. Heck, after a liaison I'll often make the same kind of roll to determine my emotional state after, on the continuum from extremely negative to infatuation.

Back It Up! Real Play Example

For example, I was playing a female cleric of Sarenrae in the Curse of the Crimson Throne Pathfinder Adventure Path. As we adventured, we met fan favorite NPC Laori Vaus, a cute bubbly (and evil) recurring character. "Tee hee, aren't decapitated heads so cute?" Our group took to her, and my PC did as well (she'd been travelling around exclusively with a bunch of extremely un-fun men for a while.) Laori and her fellow death cultist buddies, who we not entirely affectionately referred to as "the Boner Squad," popped up a couple times in the campaign as temporary allies of convenience. One day, the GM tells me that Laori propositions my cleric! Well, a flood of conflicting thoughts run through me, both in character and metagame. "I'm a good girl... But I don't think Sarenrae's faith is against it per se... It'd be an interesting twist... But it might be seen as exploitative by the other PCs... She's hot and fun... But evil..." Deadlock. So I quickly set a target and roll it. Fail by 2. I decide that my character politely demurs, but isn't completely against the idea if Laori follows up further... It didn't get a chance to go further because the inevitable Boner Squad betrayal came soon after, but I was happy with the die roll solution letting me move forward quickly and add some realistic unexpectedness to the game.

1. Make sure it's what you want

As others have stated, seducing player characters is a big departure from how most groups play D&D. Before you proceed with any of this, make sure everyone at the table is on board with this change in play style, and the possible shift in focus - away from fighting monsters and towards inter-character drama - that comes with it.

From my personal experience, a game like this can be a lot of fun if you have a mature and comfortable group, but it's definitely more challenging.

2. Interpersonal tools before game rules

It's good practice anyways, but especially if you're going toward a playstyle that allows players to significantly affect each other's characters, agree on some ground rules first, and define veto rights. Some examples:

• 
  The "X" card. Basically: "I'm not okay with where we're going, let's agree that this does not happen." Jump back a scene or two and move on in a different direction.


• 
  Fade to black. Let's assume it happened, but skip the scene. If the event is referenced later, don't describe it in detail.

If a player uses their veto, don't argue. If necessary, take a short break, otherwise focus on how to continue play in a way that's comfortable for everyone.

3. Rules should give incentives, not force behaviour

D&D is not the most ideal system if you want to focus on this, but it's not a lost cause. And if you're only making occasional social checks on player characters, it works just fine.

Fisrt off, I'd recommend to retain player agency as much as possible. Give roleplaying cues and possibly mechanical effects, but leave it to the player to decide how exactly their character reacts. Aced an "intimidate" check? Think "That half orc is fearsome and you can't help but wonder if you should really oppose them. You have disadvantage on rolls against the character", rather than "You now have to do as they say."

Look at how D&D handles mind-affecting spells for guidance. Also keep in mind that not every action has a chance of success. If your argument doesn't make any sense to the character or they're just not attracted to whoever's trying to seduce them? No point in rolling.

Another thing that I like to do but would require a bit of homebrewing in D&D is rewarding players for taking risks or allowing their characters to "succumb to tempation". You might, for example, consider offering players inspiration if they go along with what's requested of their character (and the other side rolls well). If they refuse, nothing bad happens. If they agree, they get a small mechanical benefit in exchange for their character being "manipulated". (Fate, for example, implements this pretty well.)

Final thoughts

In my opinion, most of what I've written here about social skill checks applies just as much for physical violence, which is a core element of D&D and well supported by the rules. Some players are not ok with their characters being seduced or intimidated, I know others that would object to them being grappled, restrained and cut to pieces. Many will be ok with a lot if it's only handled superficially, others might be triggered by something mentioned in passing. Know your fellow players and agree on what's ok for the group.

I agree with what others have said regarding consent-- this starts leaning into some weird territory with that. I'm also of a different school of thought than your DM, based on what you've said here, but here's my two cents:

I tend to lean towards the notion that persuasion/intimidation checks are only for things that might have been on the table for a person to do already. You can't talk someone into jumping off a bridge, for example, unless they were already considering it, you can't talk a black dragon into just not being mean anymore. Spells like Suggestion exist for a reason, and that reason is to make people do things that they are fundamentally unwilling to do. If having sex with a lady just because she's there and charming goes against the fundamental tenets of your character's personality, then she'd be out of luck at my table, no matter how many 20s she rolls. Otherwise, charm and persuasion spells become completely mechanically useless.

I would compare this situation to a hypothetical situation where your paladin tried to get her to stop doing amoral rogueish activities. It's unlikely to work, without some sort of extended roleplay that changes her character, because it seems to be pretty central to who she is as a character; same thing applies to the flip side.

To answer the question directly, there aren't any such rules in D&D, and attempts to do so will invariably run afoul of many of the objections outlined in the other answers regarding player agency.

HOWEVER...

The important part isn't really whether there are rules for it. That's easy enough to house rule on the fly.

The important part is what the stakes are that the proposed roll is going to address.
After all, seduction is not sex, even if that is often part of the desired result. Seduction, like diplomacy, or intimidation is a means to get something.

What does the rogue want out of the situation?

• Does she want to knock boots, and have nothing else change?


• Does she want to start an actual relationship?


• Does she want to get on the paladin's good side, in order to get something (information, object, favor, etc.) from him?

How does the paladin view that?

• If she wants to knock boots, would that be a distraction, or welcome relief and relaxation?


• If she wants a relationship, how does that tie into or interfere with his oath of vengeance? Would it provide a new ally directly invested in achieving that vengeance, or a loved one who might temper that desire, preventing more extreme means of achieving it?


• Is he willing to give up the desired 'thing'?

The best way I've seen to deal with inter-player social conflicts is to allow the players OUT OF CHARACTER to negotiate, between themselves, the stakes of the roll.. (Said negotiation may even obviate the need for the roll, if the players come to a mutually acceptable end result.)

If the Player doesn't trust his own decision

If I was playing the paladin, I'd want him to be resolute and never give in. On the other hand, he's only human, and if he does succumb it wouldn't really be out of character. In this case -- player vs. his own character -- I'd think it would be fair to let a roll decide the outcome.

I like Dan B's idea of letting the PC set the difficulty level. (Or asking the DM to do it.)